Filter cylinder with drainage surface

ABSTRACT

A drum filter is characterized in that its filtrate compartments (20) comprise a bottom part (28,30) and a side surface (32), which is provided with a lip or curved portion (34) extending towards the inside of the compartment (20) from the edge of the side surface opposite the bottom part. The lip (34) prevents the filtrate from escaping from the compartment (20) back through the wire surface, when the drum filter (20) is rotated.

FIELD OF THE INVENTION

The present invention relates to a rotary drum filter for treating pulp,which is especially suitable for treating fiber suspensions in the woodprocessing industry. The filter drum or cylinder is particularlydesigned for washing or thickening said suspensions, by filteringliquids from fiber suspensions.

BACKGROUND OF THE INVENTION

The invention relates more specifically to a so called rotary drumfilter, the basic construction of which comprises a tank, containing thesuspension being thickened and a wire-surfaced cylinder rotatablymounted in the tank. The ends of the drum or cylinder are sealed so asto prevent the cylinder from being filled with suspension. The cylinderhas under its wire surface filtrate compartments, into which liquid fromthe suspension in the tank is pressed due to the hydrostatic pressure,whereby the fibers in the suspension are transferred with the liquidonto the wire surface and are thickened forming a fiber mat thereon. Thecylinder in this kind of filter based on hydrostatic pressure can besubmerged in the suspension in the tank only slightly below the axialplane of the cylinder. When the cylinder rotates slowly, approximately 1to 3 rpm, a fiber mat is gradually deposited on the wire surface. Theformation speed of the mat naturally depends not only on the thicknessof the deposited mat, but also on how deep in the vessel the filtrationtakes place. The filtration reaches its maximum speed slightly beforethe bottom dead center, because, at this point, the hydrostatic pressureis close to its maximum and the build-up of a thick mat does not yetdisturb the filtration. After the bottom dead center, the formation ofthe mat slows down, until the formation, of course, ceases completely,when the mat emerges from the suspension.

As the fiber mat emerges from the tank upon rotation of the cylinder thefiltration based on gravity begins, liquid flows slowly due to theeffect of the gravity through the fiber mat into the filtratecompartments inside the wire surface. The fiber mat is removed from thecylinder at a stage, when it has passed the top dead center of thecylinder and again approaches the surface of the suspension. The mat maybe removed either mechanically with scrapers or by injecting either aliquid or compressed air through the wire surface. The liquid which hasflowed into the filtrate compartments may be discharged from theapparatus, for example, via the hollow shaft of the apparatus or byutilizing some other arrangement. The length of a filtration cycle basedon hydrostatic pressure in an apparatus according to the abovedescription is approximately 140° of a revolution of a cylinder and theproportion of the initial filtration is approximately 30° and theproportion of the vacuum drying (above the liquid surface of the tank)is approximately 100°.

The above-described basic type of a drum filter has been considerablyimproved over the decades. One significant improvement worth mentioningis the subjecting of the suspension in the tank to a suction appliedthrough the compartments, whereby the filtration of liquid isconsiderably improved and thus the capacity of the apparatus issignificantly increased. Generally, the suction operation is carried outby a suction leg in such a way that each of the filtrate compartments isconnected by means of a separate filtrate tube to a valve which isconnected to the suction leg and the shaft of the apparatus. Theoperation of the valve is controlled in such a way that the fiber mat issubjected to suction over a sector of approximately 240°. Approximately30 filtrate compartments are arranged within the cylinder, each of whichcompartments has been connected by a separate filtrate tube to the shafttube of the cylinder surrounding the valve. Since the fiber matgenerated this way is significantly more tightly attached to thefiltrate surface than the mat generated in a conventional way, thethickened fiber mat must be removed from the filtrate surface byseparate means. At this point, it should be noted that the suction legmay be replaced by some other apparatus creating subatmospheric, such asa vacuum pump.

The filtrate compartments in accordance with the prior art have onedisadvantage, which occurs particularly in the operation stage of thefilter cylinder, when the pulp web is removed from the wire surface ofthe cylinder. At this point the filter cylinder described in, forexample, FI Patent 55538 and the filter compartments therewherein haveturned, in the mat removal stage to a position wherein the filtratestill present in the compartments, will flow, in the rotationaldirection of the cylinder, toward the front wall as it is unable to exitfrom the compartment, since the discharge openings of the compartment donot usually extend along the entire width of the bottom of thecompartment. Consequently, especially when the suction is interruptedand the compartment turns further, the filtrate begins to flow againstthe wire surface, whereby it, of course, permeates the wire surface andflows back to the tank or to the pulp web to dilute the consistencythereof.

SUMMARY OF THE INVENTION

It has been possible to solve the problems of the prior art apparatuseswith the present invention in such a way that by providing the filtercylinder in accordance with the present invention it is possible toremove all liquid from the filter compartment without the risk of thefiltrate flowing back to the pulp web.

The filter cylinder in accordance with the present invention ischaracterized in that the filtrate compartments comprise a bottom partand at least one side wall, the leading side wall of each compartmentwith respect to the direction of rotation of cylindrical part isprovided at its upper edge remote from the bottom part with a curvedportion extending towards the inside of the compartment or with aprojecting lip or other overhanging portion, which curved portion or lipprevents the filtrate from escaping from the compartment back throughthe wire surface, when the cylinder is rotated.

BRIEF DESCRIPTION OF THE DRAWINGS

The filter cylinder in accordance with the present invention isdescribed in more detail below, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is an axial sectional view of part of a filtration cylinder inaccordance with the present invention; and

FIG. 2 is an enlarged fragmentary detailed sectional view at a radiallevel of the filter cylinder in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT

According to FIG. 1 a filter apparatus mainly comprises a tank 10, whichmay be either completely open from the top or covered with a hood, and acylindrical filter cylinder 12, which is arranged on shaft 14, which ismounted at its ends with bearings and seals 16 relative to the tank 10.The body of the filter cylinder 12 comprises end plates 18 attached toan extension of the shaft 14, which plates at the same time prevententry of the suspension to the inside of the cylinder, and filtercompartments 20, by means of which the ends of the cylinder areconnected to each other and which form the primary structure of thecylinder. The cover of a filter compartment may be either a perforatedplate, on which the wire surface serving as a filter surface is located,or the wire surface 22 itself. If the cover of the filtrate compartmentis a perforated plate, it will also serve as a member bearing thestresses of the cylinder. A great number of filtrate compartments 20,approximately 30-40, has been arranged at the periphery of the cylinder.The bottom surfaces (30, 28) of the filtrate compartments 20 areinclined either only towards one end of the cylinder or from the centerin both directions towards the opposite ends thereof, mainly along theentire length of the cylinder, which, in most of the cases, is more than5 meters. Filtrate tubes 24 leading towards the shaft 14 of the cylinderare connected to the bottom surface 28 of the filtrate compartments 20at the end of the cylinder (or at both ends thereof, when the bottomsurfaces of the compartments are inclined towards both ends). Adjacentto the filtrate tubes 24 is a distribution chamber 26, to which thefiltrate tubes 24 are connected in either one or two rows mainlyaccording to the diameter of distribution chamber 26. Distributionchamber 26 is mainly required for guiding the filtrate flowing from thefiltrate tubes to the suction leg without the need to constrict thesectional area of the flow. In the distribution chamber the sectionalarea of the flow is changed from the shape of a round filtrate tube to arectangular one defined by the radial intermediate and end walls of thechamber. A valve apparatus 40 for guiding the filtrates of tubes 24 in adesired manner is located inside the distribution chamber 26. By meansof the same valve it is possible, during the operation of a conventionalfilter, to also conduct liquid or gas for the removal of the fiber matthrough filtrate tubes 24 to filtrate compartments 20.

Filtrate compartment 20 comprises, according to FIG. 2, a bottom part 28substantially parallel to the periphery of the filter surface, to whichpart 28 a filtrate tube 24 is connected. An inclined bottom part 30 isjoining the edge of the bottom part 28, and extends in the rotationaldirection (arrow B) of the filter cylinder and advantageously forms anangle of approximately 165° with the bottom part 28. A substantiallyradial wall 32 is joining the front edge of the bottom part 28 in therotational direction of the drum filter. The outer edge of wall 32 isprovided with an inwardly extending lip or curved portion 34 pointingtowards the filtrate compartment, the purpose of which curved portion 34is to prevent the filtrate in the compartment from escaping through afilter surface 22 back to the fiber mat or, after the mat is removed, tothe tank when the cylinder turns to a position, in which the pulp web isremoved from the filter surface (rotated approximately 60° downwards andto the right from the position shown in FIG. 2). The bottom part 30extends upward advantageously to about half the height of wall 32 of thenext filtrate compartment. Bottom part 30 is attached to wall 32, forexample, by welding. The construction thus produced is very resistantagainst different stresses it may be subjected to. Additionally, curvedportion 34 will strengthen the compartments against bending stresses.Curved portion 34 also forms a suitable location for attachment offilter surface 22.

Curved portion 34 formed in the filtrate compartment 20 will retaininside all of the filtrate which might otherwise drain back to the pulpweb during the removal stage of the web. The filtrate will remain in thecompartment 20 until the compartment submerges under the surface of thesuspension and the filtration based on the hydrostatic pressure begins,at which point the filtrate is either discharged with the air from thefiltrate tubes or during the stage when suction is applied.

As it will be appreciated from the above description, it has beenpossible to eliminate or minimize the defects of the prior artapparatuses by providing an apparatus, which is easy to produce and bymeans of which the filter cylinder becomes stronger than those of theprior art.

What is claimed is:
 1. A filter cylinder rotatable in a tank andincluding a substantially cylindrical member having an axis and beingcovered by a filter surface, said filter cylinder comprising a pluralityof adjacent axially extending filtrate compartments below said filtersurface, each said compartment comprising:a bottom part spaced from saidfilter surface; a filtrate impermeable continuous side wall, which, whenviewed in the direction of rotation, is a leading sidewall, having anouter edge and extending from said bottom part toward said filtersurface and in contact therewith; a protrusion extending at an anglerelative to said filter surface from said outer edge inwardly towardsaid bottom part of said compartment, terminating within saidcompartment; and an opening in said bottom part, facing said filtersurface, between said leading sidewall of said compartment and a leadingsidewall of an adjacent compartment of said filter cylinder.
 2. Thefilter cylinder of claim 1, wherein the leading side wall extends in asubstantially radial direction and said bottom part of said filtratecompartment further comprises a first and second plane, said first planeextending from said substantially radially extending leading side wallsubstantially parallel to said filter surface and said second planebeing tapered by extending from said first plane toward said filtersurface.
 3. The filter cylinder of claim 1, additionally comprising ashaft co-extensive with said axis and a plurality of filtrate tubescommunicating with said compartments in the vicinity of said leadingside wall; and wherein the distance between said bottom part of saidfiltrate compartments and said shaft decreases in the direction of saidfiltrate tubes.
 4. The filter cylinder of claim 1, wherein said leadingside wall has a height; and wherein said protrusion extends towards saidbottom part for a distance less than said height.
 5. The filter cylinderof claim 1, wherein said compartment additionally comprises an openingfacing said filter surface and a trailing side wall; said protrusionhaving an axially extending edge spaced from said filter surface; andwherein the distance between said edge and said trailing side walldefining said opening of said compartment and the distance between saidleading side wall and said trailing side wall is less than 1:4.